This invention relates to a synchronism establishing method and apparatus.
Generally, in various systems for signal processing such as image signal processing or audio signal processing employed in communication in which a plurality of processes are achieved by a combination of several modules to extract a desired function as an apparatus or a unit, particularly, for example, in TDM (time division multiplexing) multiplexing apparatus or multi-media communication apparatus which employ digital circuits or multi-media switched networks as represented by a digital exchange or an ATM (asynchronous transfer mode) exchange, it is necessary to establish, on the receiver side, synchronism with the transmitter side.
Conventionally, when various media are involved in input signals, for example, for signal compressing processing of speech, a plurality of signal processing modules are used for the media. Also a determining section is used which analyzes a characteristic of an input signal to select a necessary signal processing module by means of some apparatus such as a service trunk in an exchange.
Accordingly, the kind of a medium inputted is identified and an optimum one of the signal processing modules is selected by the determining section. The result of processing of the selected processing module (information to be transmitted) is forwarded onto a transmission line to transmit it to the receiver side by switching control based on a result of the determination by the determining section. Meanwhile, some communication means is used to notify the receiver side of the result of the module selection.
On the receiver side which receives the information, decoding processing is performed and a reproduced signal is outputted by a module selected based on the information regarding the selected module.
However, normally a signal processing module in most cases effects encoding conforming to a multiplexing hierarchy of a communication apparatus (bearer rate), and sometimes it cannot assure a band for the notification of a result of determination thereof within a band of a channel allocated to it.
One alternative in this instance is transmission of switching information of a module making use of a control communication path which is assured for control between different communication apparatus. However, normally the control communication path does not assure a sufficient band to transmit information of the channel level on a real time basis. Also, from the point of view of the processing capacity of a control processor, it is difficult to apply the control communication path to any other processing than comparatively low speed processing such as switching of a call by voice.
Therefore, signal processing modules must be switched on a real time basis within a connected call in service. Besides, since it is difficult to transmit switching information between modules in a band allocated to the channel, it is necessary for the receiver side to obtain frame synchronization information in order to establish synchronism between processing cycles of modules on the transmitter side and the receiver side.
From the situation described above, a synchronism establishing method is conventionally performed using, for example, such a synchronism establishing apparatus as shown in FIG. 13 in order to transmit frame synchronization information.
Referring to FIG. 13, the conventional synchronism establishing apparatus shown is incorporated in a signal processing unit of a signal processing system and includes a facsimile (FAX) signal detecting codec 101 connected in parallel to a speech codec 102 and an echo canceller (ECAN) 5. Either one of the facsimile signal detecting codec 101 and the speech codec 102 is selected by a pair of selectors 3 and 4.
The facsimile signal detecting codec 101 and the speech codec 102 are connected to a multiplexing unit (MUX) not shown by way of an interface circuit 6 and also to a terminal not shown by way of another interface circuit 7.
Here, the facsimile signal detecting codec 101 and the speech codec 102 are individually used as modules which have independent synchronism establishing means. In this manner, normally two or more modules of different types are provided in combination in a signal processing unit of a signal processing system. Further, the facsimile signal detecting codec 101 and the speech codec 102 are switched so as to alternatively operate to effect transmission of a signal in accordance with the necessity.
When the signal processing unit shown is used as the master side apparatus, each of the facsimile signal detecting codec 101 and the speech codec 102 effects its respective processing operation when it detects establishment of synchronism based on its own frame pattern.
The facsimile signal detecting codec 101 detects a facsimile signal from within a signal transmitted thereto and decodes the signal when the signal is a coded signal, but encodes the signal when the signal is a decoded signal.
The speech codec 102 includes a speech encoder 102a and a speech decoder 102b which encode and decode a speech signal, respectively.
The synchronism establishing apparatus constructed from the modules described above operates in the following manner. In particular, a synchronization pattern, a period and so forth are switched between the modules to effect transmission or reception of a signal, and on the receiver side, it is determined by which module the signal can be pulled in in response to the module selection on the transmitter side. Then, a processing operation using the selected module itself is performed. In short, processing for establishment and protection of synchronism is performed individually among the modules.
Since the conventional synchronism establishing apparatus basically requires a combination of modules developed individually in this manner, where several existing modules are present, necessary functions can be realized comparatively easily.
While the conventional synchronism establishing apparatus basically requires a combination of modules developed individually, it has a problem to be solved that involves, upon switching between modules, a switching transition process having a time length corresponding to the number of protection stages necessary for pull in and protection of synchronism.
It is another problem to be solved that, due to the first problem described just above, an undefined state of a period equal to the frame period multiplied by the number of protection stages for establishment of synchronism is involved, and some masking processing is required such as stopping a decoded signal output.